Solar energy for Earthbound applications continues to be an area of intense design effort because of its environmental friendliness. In space-based applications, the abundance and permanence of sunlight makes solar energy particularly attractive.
Broadly speaking, two solar-cell-based systems exist today for generating electricity from light. The first takes the form of flat panel solar arrays. In such arrays, light (typically received from the Sun) impinges directly upon solar cells in the array, without concentration or other amplification.
The second system generally in use is called a concentrator system. In concentrator systems, optical elements concentrate light before it impinges on solar cells. The degree of concentration is frequently quantified in relative terms as a multiplier (such as "10X") or in absolute terms as a number of "suns" (the exact meaning of which is well-known to those skilled in the art). One type of concentrator system is a "line-focus" concentrator system and is characterized by a line-focusing lens, such as a Fresnel lens, that concentrates light received into its area into a narrow ribbon. The concentrated narrow ribbon of light impinges upon an elongated solar cell or linear array of such solar cells.
Solar cell technology is also increasing in sophistication.
One example of a new breed of solar cell is a monolithic multi-junction ("MMJ") cell. "Monolithic" means that such cells are formed upon or within a single, semiconductor substrate, which may be composed of gallium arsenide ("GaAs") or germanium (Ge), much as today's integrated circuits are structured. "Multi-junction" means that the cell is formed in multiple layers ("junctions") within the substrate. Each of the junctions is optimized (by means of a doping process) to absorb light in a given range of frequency. Light that is not absorbed in upper junctions is transmitted to lower junctions for absorption there. Ideally, the junctions in an MMJ cell cooperate to absorb as much energy as physically possible within a given spectrum of light. Most MMJ cells have two junctions, although cells having higher numbers of junctions are now beginning to appear in commercial form. The junctions are coupled together in electrical series to deliver electricity at a single output.
A problem has arisen in pairing line-focused lenses with MMJ cells. The problem stems from chromatic aberration in such lenses that tends to separate the colors in broad-spectrum light (so-called "chromatic dispersion"). Chromatic aberration causes light to be projected onto the MMJ cell in a frequency-dependent manner, degrading the power output of the cell.
The problem can best be illustrated by way of example. If light having laterally dispersed blue (higher frequency) and red (lower frequency) components is projected onto an MMJ cell, the junction optimized for absorbing blue light would respond with electrical current in a different lateral location than would the junction optimized for red light. Because the junctions are coupled in series, the differing junction current locations require lateral currents to flow through the layers between the junctions. This gives rise to two problems. First, the layers between junctions present a significant lateral sheet resistance, impeding conduction of the inter-junction currents dissipating power within the cell. This decreases the efficiency of the MMJ cell significantly. Second, the resulting power dissipation in the cell raises the cell's overall temperature perhaps, in localized spots, to the point that the cell is damaged or destroyed.
Therefore, what is needed in the art is a way of increasing the power output of MMJ cells in concentrator systems.